Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. Most of the "direct" thermographic recording materials are of the chemical type in which upon heating an irreversible chemical reaction takes place and a coloured image is produced.
EP-A 248 405 discloses a thermal recording material with a colour-developing layer, which contains an electron acceptor and an electron donor in addition to the usual additives, characterized in that the electron acceptor is a metallic double salt of a long-chain fatty acid with 16 to 35 carbon atoms and the electron donor is a polyhydric aromatic compound of formula (I): ##STR2##
wherein R is an alkyl group with 18 to 35 carbon atoms, ##STR3##
wherein R.sub.1 is a C.sub.18 - to C.sub.35 -alkyl group, n is an integer of 2 or 3, --X-- is --CH.sub.2 --, --CO.sub.2 --, --CO--, --O--, --CONH--, --CO(R')N-- (wherein R' is a C.sub.18 - to C.sub.35 -group), --SO.sub.2 --, --SO.sub.3 -- or --SO.sub.2 NH--. Example 4 concerns a thermosensitive recording material with a colour-developing layer containing silver aluminium double salt of octadecyl 2,3,4-trihydroxybenzoate.
EP-A 599 580 discloses a thermal recording sheet comprising, in order: (a) a substrate; (b) an intermediate layer which comprises a pigment having an oil absorption according to Japanese Industrial Standard (JIS) K501 of 100 mL/100 g or less; and (c) a thermal color developing layer which comprises a leuco dye type chromogenic component consisting of a leuco dye and an organic color developer and a metal chelate type chromogenic component consisting of an electron acceptor and an electron donor, wherein: the organic color developer is at least one of compounds of formula (I) and formula (II): ##STR4##
wherein R is propyl, isopropyl, or butyl; ##STR5##
the electron acceptor is a metal double salt of a fatty acid having 16 to 35 carbon atoms; and the electron donor is a polyhydric aromatic compound of formula (III), which is the same as formula (I) of EP-A 248 405.
JP 09142029 discloses a recording layer containing at least one kind of aminobenzene sulfonamide derivative shown by formula I, at least one kind of multi-value phenolic compound, shown by formula II [same formula as formula I of EP-A 248 405 wherein X.dbd.--CH.sub.2 --, --CO.sub.2 --, --CO--, --O--, --CONH--, --CON(R')--] and a high class aliphatic metallic salt, is formed on the supporting body of a heat sensitive recording body.
The thermosensitivity of organic silver salt/reducing agent systems is dependent upon the choice of reducing agent. However, increased thermosensitivity is generally associated with poorer image gradation i.e. dependence of image density upon applied thermal energy, reduced stability and poorer image colour. There is therefore a need for reducing agents which increase the thermosensitivity of organic silver salt/reducing agent systems without substantially affecting the gradation, stability and image colour of prints made therewith.